Sussex2 - its not reporting for duty that needs the flexibility - its going home after. At present it is perfectly feasible to book a standby on a flight leaving say 90 minutes after scheduled arrival into T5. If you don't make it, well bad luck but no real stress, just rebook onto the flight 3 hours later.
However, can you risk missing that flight and losing your entire fare, plus paying for another non-flexible, short notice ticket if your flight/stand/bag is delayed by only 20 minutes?
These days quite a few commuters buy full(ish) fare Hotlines to get to work, but Standby tickets to get home for that exact reason. Have you seen the price of a LON-BCN single fare 2 hours pre departure?